This inverter came to me for repair from
a fellow HRSA member who is an enthusiast for 32V lighting plants and their
accessories. Previously, I had repaired this
AWA solid state inverter for him. The Van Ruyten vibrator inverters
were popular, and a 12V 100W model in my collection is described here.

The 58TV is a 32V model, with an output
of 230V 50c/s at 200W. The power rating and the "TV" designation in the
model number perhaps suggests the inverter was intended to power television
receivers. 230V was the standard mains supply in Victoria where this inverter
was manufactured. There were many areas outside the cities reliant on 32V
lighting plants which could receive TV signals, so there was a market for
32V television sets, and inverters to operate conventional 240V sets. Ferris
manufactured 32V sets, but for other brands, an inverter had to be used.

This 58TV was obviously well used. Along
with it came a box containing three extra vibrators and the suggestion
that some were faulty or unknown in condition.Two 100W transformers are connected in
series to obtain the 200W rating. This is a better method than connecting
transformers in parallel because series connection will guarantee equal
current sharing in the windings. Needless to say, the inverter is rather
heavy!

Above chassis has only the transformers and vibrator visible.

As per the 12V model, a large UX-5 based
vibrator is used, which appears to be of ATR (American Radio & Television)
design. The vibrator is identical to the 12V type except for the drive
coil. It is a dual-interrupter type with two sets of primary switching
contacts connected in parallel inside the vibrator. The vibrator is a series
drive type with a separate contact for the drive coil. The vibrator is
labelled as being made by Liebmann Clarke Pty. Ltd. in Richmond, Victoria.
It has no type number, except for the voltage rating.

Under the chassis with original components.

The Circuit.

Transformers.The most unusual thing perhaps is the
two series connected transformers. These are both Trimax type TP3640, rated
at 100W for 50 cycles.It is unclear if there is a 100W model
using just one TP3640. Are these just 32V transformers connected in series?
Given that the primary voltage is halved when two are in series, the secondary
voltage would also be halved (i.e. 115V). But, because the secondaries
are in series, the final output is 230V. Or are they actually 16V transformers
with 115V secondaries? Either way the circuit would work, but it's possible
the regulation would be poorer if they were 32V types - this being because
the turns per volt ratio would be more than optimum. One could test the
transformers individually at both voltages - if they saturated at 32V,
it would indicate they were most likely 16V types.Conveniently, the terminals for the transformers
are labelled which made tracing the circuit relatively easy. The
start, finish, tapping, and shield connections are all identified.

Transformer connections are clearly labelled.

In view of the power rating, a high - low
switch is provided which selects the full secondary, or a tapping thereof.
This is necessary because regulation is not perfect, and also to allow
for variations in the supply voltage.

Output Circuit.This consists of a secondary buffer (0.56uF
600V) and a balanced pi-filter with two chokes and four 0.1uF condensers.
These condensers also add an extra 0.1uF to the secondary buffer. The earth
pin on the output socket was not connected to anything.

Primary Circuit.This is quite conventional for this kind
of circuit. The vibrator reed is fed with 32V and this switches alternately
to either side of the transformer primary. The centre-tap is earthed. This
is the reverse of car radio practice where the C.T. is the supply, and
the vibrator contacts switch either side of the primary to earth.The main buffer is 8uF, which consists
of four parallel 2uF capacitors. For inverters operating above 12V, it
becomes practical to have most or all of the buffer capacitance across
the primary, which is electrically the best place for it. Additionally,
there are four 1uF capacitors; two from either side of the primary to earth,
and two across the vibrator contacts. These latter four are for RFI and
spark suppression, but will also add to the primary buffer capacitance.The vibrator drive coil contact also requires
a spark suppression circuit. This consists of a 0.1uF and 50R across the
contacts. The 50R limits the capacitor discharge current when the contact
closes. Oak radio vibrators, although series drive, do not require this
circuit as they use a patented short circuited secondary winding on the
drive coil, which slows the rate of flux collapse when the contact opens.

Input filtering for the 32V DC supply is
provided by two chokes and a pair of 1uF condensers. A normal domestic
tumbler switch turns the inverter on or off. An inline 10A fuse provides
protection from overload.As the inverter uses a non synchronous
vibrator, and there are no polarised components, it does not matter which
way round the inverter is connected to the 32V supply, except for the matter
of earthing. One side of the supply connects to the inverter chassis and
case, and this should be the earthy side of the supply for best RFI suppression
and to prevent short circuits.

Restoration.The first thing to do of course is replace
all the paper condensers, except for the two 1uF's in the 32V DC input
filter. Any leakage here is unimportant.Unlike 12V inverters, the condensers in
the transformer primary circuit of a 32V inverter are under a good deal
more stress, and any leakage will cause problems. There is about 134V peak
to peak across the combined transformer primaries. Indeed, it was noted
there was already a bulge in the casing of one of the primary buffers.
It was also noted that the 10A fuse was blown, and when the vibrators were
examined, it could be seen that the inverter has had a hard time, and not
necessarily used under the correct conditions.

New capacitors installed.

The primary buffer was a bundle of four
parallel connected 2uF AEE Microcaps rated at 300V. I replaced this with
a single 8.2uF polypropylene 250V type. This was an expensive capacitor,
but is ideally suited to the pulse current it has to carry, as well as
being neater than four individual capacitors. The rest of the capacitors
were replaced with ordinary polyester types.Experience has taught me that the choice
of buffer capacitance is not always as good as it should be in the case
of DC-AC inverters, so before simply replacing the 0.56uF secondary buffer
with the same, I checked the waveform.

Unreliable leaky paper capacitors and a blown fuse were replaced.

Out of the four vibrators supplied, one
looked reasonable enough for testing without repair. At this point I was
curious to see what the waveform would be. This was observed across the
primary buffer condenser, since this is what the vibrator contacts see.
Tests were done with no load, a 100W incandescent bulb (resistive load),
and a 40W fluorescent lamp with no phase correction (low power factor inductive
load).

No load waveform. The short slope indicates excess buffer capacitance.

As mentioned in the other DC-AC inverter
articles on this site, buffer (timing) capacitance for a general purpose
inverter is difficult to choose, and usually ends up being a compromise.
The problem is that the load may be capacitive, resistive, inductive, or
only draw current at the voltage peaks. This variation in load affects
how the timing capacitance works, which subsequently determines the timing
of current flow vs. when the vibrator contacts close and open. And, this
ultimately determines vibrator life.

Loaded by a 100W incandescent bulb, the waveform is as expected
with the buffer capacitance now appearing to be inadequate. Contact bounce
is visible - the contacts had not been cleaned at this stage.

If the load is resistive, as in the case
of an incandescent light bulb, the buffer capacitance is effectively reduced
because current in the load, and thus the primary circuit, is always flowing,
and the buffer capacitance is discharged faster than normal.The loading prevents destructive overshoot,
so transformer insulation is not at risk, and excessive contact sparking
does not occur. However, because the timing circuit is no longer operative,
the vibrator contacts are carrying more current than the ideal situation
at the point of closing and opening. With an ideal buffer circuit, current
flows only after the contacts have closed, and stops just before they open.

With a low power fluorescent lamp, the buffer capacitance was clearly
inadequate with undesirable overshoot appearing.

If the load is inductive, as with a fluorescent
lamp which is not power factor corrected, or an induction motor, the buffer
capacitance is cancelled out to some degree, and destructive overshoot
appears on the waveform, as does arcing at the vibrator contacts. This
is shown here quite clearly.Purely capacitive loads are unlikely to
be used with an inverter of this kind. The only common instance with domestic
appliances is where a capacitor is used as a voltage dropper. The effect
of using a capacitive load is that the timing capacitance is increased,
with an increase in primary current.

Inadequate buffer capacitance is much more
damaging than too much, so given the range of loads the inverter may be
used with, the compromise is to provide just enough capacitance for the
inductive loads likely to be used. This unfortunately results in an increase
in current consumption with other loads, and is particularly evident with
no load, or very low loading. Higher loads tend to swamp this out. Therefore,
an inverter like this should not be used for small loads to obtain best
efficiency.

I thought it worthwhile to connect the
output socket earth pin to the chassis. Even though it is not required
for safety since the output is balanced, it's a good idea to improve RFI
suppression, where the equipment powered is fed with a three core cable.

The Vibrators.In each of the vibrators, the foam rubber
insulation had decomposed, leaving a sticky substance on the vibrator frame.
The problem is when some of this gets into the contacts. Aside from that,
the vibrator mechanism will rattle against the can.

Decomposed acoustic insulation.

Foam rubber crumbling apart. Note the misalignment of the upper
set of contacts.

Cans re-lined with modern foam rubber.

Restoring the vibrators was the most time
consuming part of the job. Some of them had the contacts in obvious misalignment,
as well as being badly damaged. One vibrator had short circuited contacts.
Then was the matter of contact timing. Not only do the contacts have to
be set for the right duty cycle, but also both sets of contacts have to
be timed to make contact simultaneously. It was determined that the VR
vibrators operate with a 70% duty cycle. Effectively, this means an on
time for each contact of 7ms, and an off time of 3ms.

Vibrator 1.

The first vibrator was in reasonable condition
and just needed a physical clean. Both contacts have a 6.9ms duty cycle.

Vibrator 2.

Likewise, the second vibrator needed cleaning.
It has a duty cycle of 7.1ms on one side and 7.4ms on the other.

Vibrator 3.

The third vibrator was in very poor condition.
I noticed while connected to the test
panel that vibration seemed restricted and one set of power contacts
were sparking - even though nothing was connected to them!Note the upper contacts - the tungsten contact has been welded off
centre on the lower side contact arm. The lower contacts are also misaligned.

Looking at the other set of contacts, the excessive misalignment
is obvious on the lower set. They were actually touching!

A lump of solder was shorting between the frame and one set of power
contacts. It was quite difficult to remove. A smaller lump of solder is
visible on the adjacent side towards the centre.

The strange performance turned out the
be a lump of solder stuck between the frame and one of the power contacts,
effectively turning the vibrator into a shunt drive type, which it was
not designed to perform as. Once this was removed, normal vibration was
returned. Next, the stack bolts had to be loosened and the contacts re-aligned.
One of the tungsten contacts had been welded off centre to its support
arm. Nothing could be done about this, but the corresponding contact had
enough adjustment to be aligned with it. Once all that was done, the contacts
were cleaned and the duty cycle adjusted. It turned out well with an equal
7ms per contact duty cycle. However, once reinstalled in the can, vibration
seemed weak. This was because the reed starting voltage was too low. The
adjustment for the reed contact is a compromise between starting voltage
and sufficiently vigorous vibration. It is pointless having the reed start
vibrating at say 10V when there is no way the inverter would be used on
such a low voltage. A starting voltage of about 20 gives sufficient vibration
with the can in place.

Vibrator 4.

Severely burned contact indicates incorrect operating conditions.

Part of the contact has eroded away.

The fourth vibrator had the worst contact
damage. All I could do is clean it up as best as possible. Duty cycle was
7.4ms and 7.2ms.

Despite vibrators 3 and 4 being badly damaged,
they did restore OK and performance is good. The question of how these
vibrators were assembled has me intrigued. There is no way the contacts
could have moved out of adjustment like they had - they had clearly been
used like that for the previous life of these vibrators - the wear marks
make this obvious. My guess is that whoever assembled them found their
job boring and just didn't care. Human nature being what it is - just imagine
a bored apprentice assembling vibrators all day. The fact is the vibrator
will work misaligned, but will just have a short life. And as long as any
lumps of solder aren't actually touching anything at the time of testing,
the vibrator is sealed up and sent out. I have never seen such poor quality
control in vibrator assembly, except with some Ferrocart types. And, if
I am not mistaken, it does appear that the Van Ruyten and Ferrocart vibrators
were assembled by the same company; i.e. Radio Corporation. Luckily, the
VR vibrator is of good design, and if set up properly there should be no
problems with it. Unfortunately, the result of careless manufacture only
provides negative comments about vibrator reliability from the naysayers.

There is also always the problem of unsuitable
loads being plugged into an inverter. This probably causes more vibrator
damage than anything else. Many treat the output socket of an inverter
as though it was a domestic power point, with the ability to supply 10A
and with no concern for power factor.

As can be seen, a perfect duty cycle was
not obtained for all of the vibrators. Because of the time consuming nature
of adjustment, the labour cost would start to become uneconomic for
the owner, if perfection was to be obtained. Nevertheless, the outcome
was acceptable.

High speed photography shows the vibrator in action at the instant
when contact is made. The human eye sees operation as shown at right.

As mentioned previously, these vibrators
are dual-interrupter types. So, how does one adjust each set so they close
and open at the same time as each other? In the case of vibrators with
separate connections to each contact, such as the Oak dual- interrupter
types, one can use an oscilloscope. But, with the VR type this is not possible.
My method is to adjust one set of contacts for correct operation, and then
bring in the adjustment of the parallel contacts so they spark at the same
time as the first set. To get the spark, the contacts are connected across
the output of a 30V 2A current limited power supply and the reed moved
by hand. It isn't terribly scientific, but it must be remembered that over
the life of the vibrator it is impossible to retain perfect adjustment
anyway. This is why the current rating of such vibrators is simply not
just the double of one set of contacts; the rating is somewhat less.

What to do about the Buffer.Interestingly, with the 0.56uF secondary
buffer removed, the no-load waveform was actually very good, and just what
it should be. In the real world with different loads, 0.56uF is probably
about the right amount - except for inductive loads. If the inverter was
only going to operate televisions, radiograms, and similar largely resistive
loads, the 0.56uF could be left in situ. There is however the possibility
that a fan or fluorescent lamp will be plugged in, and in that situation
there's a real risk of vibrator damage. If the fluorescent lamp contains
a phase correction capacitor, then the power factor is almost one, and
it appears as a resistive load. This presents no problems. However, many
fluorescent lamps, particularly for domestic operation, do not contain
a capacitor. They present a low power factor load which is inductive. In
this case, the buffer capacitance needs to be increased.The best compromise (short of including
a switch for two different buffer capacitors - which will no doubt be incorrectly
operated by non technical users), is to find the value of capacitance suitable
for the most powerful fluorescent lamp likely to be used, and leave it
at that.

I used a double 20W fluorescent lamp for
the tests. This is electrically the same as a single 40W lamp in terms
of current and power factor, since the two tubes are operated in series
with a 40W choke.

Double 20W fluorescent lamp provides an inductive load for testing.

It was found that 2uF of buffer capacitance
gave a very good waveform, and the inverter operated with good efficiency,
but on no load the current draw at 32V was more than desirable. 1.5uF was
an improvement for no load operation, and still acceptable. Better still
was 1uF. There is some overshoot, but not of a destructive level, and vibrator
sparking is not problematic. A desk fan also provided acceptable operation
with this value. On that basis, two parallel 0.47uF 630V polyester types
were substituted for the original 0.56uF. It was decided not use an X2
type 250V AC capacitor as the value of these tends to decrease over time
when connected across AC.It is still preferable that fluorescent
lamps be fitted with a phase correcting condenser where possible. Certainly,
this is important if fluorescent lamps of more than 40W are used. This
of course is not applicable to fluorescent lamps with an electronic ballast,
as these do not present an inductive load.

Buffer

0.56uF

2uF

1uF

No load input

600mA

1.2A

800mA

40W fluorescent (LPF)

3A

1.5A

1.7A

100W incandescent

3.2A

3.4A

3.4A

Table shows input current at 32V with various buffer condensers.

No load with 1uF buffer. Slope is noticeably short which indicates
excess capacitance.

100W lamp load with 1uF buffer. Ringing is due to transformer leakage
inductance.

40W fluorescent lamp (LPF) with 1uF buffer. Capacitance is less
than ideal, but not low enough to cause vibrator sparking.

Performance.

Van Ruyten operating an AWA D65 television set. Current draw at
32V is 5.2A.

For valve television use, the Van Ruyten,
or any other vibrator inverter is not ideal, because of the output waveform
having a reduced peak to peak voltage. The B+ supply is determined by the
peak supply voltage. In the case of the 240V AC mains supply, this being
sinusoidal provides a peak voltage of 340. A 240Vrms square wave however
provides a peak of only 240V. The output of a typical vibrator inverter
has a peak voltage somewhere in between these values. With the inverter
running an AWA D65 TV set, the output was 230Vrms with a peak of 260V.
The AWA D65 is a typical mid 1960's valve set with a solid state voltage
doubling rectifier power supply.

Had the 230Vrms been a sine wave, the peak
would be 322V. Effectively, the B+ rail voltage is reduced by 20%.(There were some vibrator inverters made
in the U.S. by Electronic Laboratories and Cornell Dubilier Electronics
which did provide a sine wave output).The set worked quite well, but a slight
lack of picture width due to the lower B+ supply was evident. It is quite
possible that there is enough internal adjustment in the set to compensate
for this, but this was not investigated. Pleasingly, there was no beat
pattern evident in the picture because of the 50 cycle output of the Van
Ruyten. This is in contrast to the solid
state AWA inverter with its 60 cycle output.

Load

Input Current @ 32V

Output (Low)

Output (High)

15W

1.1A

253V

291V

60W

2.2A

230V

260V

100W

3.2A

212V

238V

150W

5.3A

205V

226V

200W

6.9A

191V

208V

AWA TV set

5.2A

not measured

230V

A range of loads were measured to determine
efficiency and regulation. Except for the TV set, these were incandescent
lamps. The output rms voltage was measured with the output switch in both
high and low positions. Input was maintained at 32V at the supply cable.
Considering the simplicity of the inverter, regulation is quite good. As
is typical, efficiency is greater with higher powered loads.

100W incandescent lamp is an ideal load.

In summary, provided the user is aware
of the limitations, the Van Ruyten 58TV works very well, and as long as
suitable loads are used, and excessive input voltages are avoided, vibrator
life should be very long.